By way of example, defects in a ceramic substrate are described below. Various types of ceramics, including insulation ceramics and piezoelectric ceramics, are used for many machine parts as well as for electronic parts.
Cracks in a ceramic substrate diffusing light or air bubbles confined in the substrate cause its optical reflectance or transmittance locally to vary, thus making the substrate optically nonuniform. Similarly, cracks in a ceramic substrate diffusing a little light or air bubbles confined in the substrate cause its thermal conductivity locally to vary, and thus its temperature distribution becomes uneven. Minute defects at the surface of, or inside, ceramic substrates, including microcracks with an opening width on the order of submicrons, serve as start points of crack propagation, so that brittle fracture occurs. This behavior of such minute defects is a serious problem. Defects, especially crack defects at, or near, the surface of a substrate are considered to matter. The practical limit length of these crack defects is said to be 30 to 100 .mu.m.
Known methods of detecting the defects above include visual inspection using an optical microscope or the like, liquid penetrant testing, ultrasonic testing, ultrasonic microscopy, radiological testing, and infrared thermography.
Visual inspection using an optical microscope or the like, however, has a problem of low accuracy of defect detection both because the size of defects to be detected, such as cracks, is small and because light microscopes bring out unclear contrast.
Defect detecting apparatuses are required to be inexpensive, since they are used in production lines to inspect inexpensive substrates. The apparatuses are also required to operate at high speed. Apparatuses accurately performing liquid penetrant testing, ultrasonic testing, ultrasonic microscopy, and radiological testing cannot meet either of the requirements above.
When an electrically conductive object is inspected by infrared, its heating distribution is measured when energized, or the temperature distribution over its face is imaged using a two-dimensional infrared sensor when its back is heated. However, if the object has high thermal conductivity, an uneven temperature distribution at a defect disappears immediately after the object is heated. This makes it difficult to detect a defect, where temperature distribution is uneven, thus posing a problem of low defect detection sensitivity.